Genetics Problems

1. A rooster with gray feathers is mated
with a hen of the same phenotype. Among their offspring, 15 chicks are gray, 6
are black, and 8 are white.

What is the simplest explanation for the
inheritance of these colors in chickens?

What offspring would you predict from the
mating of a gray rooster and a black hen?

2. In some plants, a true-breeding,
red-flowered strain gives all pink flowers when crossed with a white-flowered
strain: RR (red) x rr (white) ---> Rr (pink). If flower position (axial or
terminal) is inherited as it is in peas what will be the ratios of genotypes and
phenotypes of the generation resulting from the following cross: axial-red
(true-breeding) x terminal-white? What will be the ratios in the F2
generation?

3. Flower position, stem length, and seed
shape were three characters that Mendel studied. Each is controlled by an
independently assorting gene and has dominant and recessive expression as
follows:

Character

Dominant

Recessive

Flower
position

Axial
(A )

Terminal
(a )

Stem
length

Tall (T )

Dwarf (t )

Seed shape

Round (R )

Wrinkled
(r)

If a plant that is heterozygous for all three
characters were allowed to self-fertilize, what proportion of the offspring
would be expected to be as follows: (Note - use the rules of probability (and
show your work) instead of huge Punnett squares)

homozygous for the three dominant
traits

homozygous for the three recessive traits

heterozygous

homozygous for axial and tall, heterozygous for
seed shape

4. A black guinea pig crossed with an
albino guinea pig produced 12 black offspring. When the albino was crossed with
a second one, 7 blacks and 5 albinos were obtained.

What is the best explanation for this genetic
situation?

Write genotypes for the parents, gametes, and
offspring.

5. In sesame plants, the one-pod condition
(P ) is dominant to the three-pod condition (p ), and normal leaf (L ) is
dominant to wrinkled leaf (l) . Pod type and leaf type are inherited
independently. Determine the genotypes for the two parents for all possible
matings producing the following offspring:

6. A man with group A blood marries a woman
with group B blood. Their child has group O blood.

What are the genotypes of these individuals?

What other genotypes and in what frequencies,
would you expect in offspring from this marriage?

7. Color pattern in a species of duck is
determined by one gene with three alleles. Alleles H and I are codominant, and
allele i is recessive to both. How many phenotypes are possible in a flock of
ducks that contains all the possible combinations of these three alleles?

8. Phenylketonuria (PKU) is an inherited disease
caused by a recessive allele. If a woman and her husband are both carriers, what
is the probability of each of the following?

all three of their children will be of
normal phenotype

one or more of the three children will have the
disease

all three children will have the disease

at least one child out of three will be
phenotypically normal

(Note: Remember that the probabilities of
all possible outcomes always add up to 1)

9. The genotype of F1 individuals in a
tetrahybrid cross is AaBbCcDd. Assuming independent assortment of these four
genes, what are the probabilities that F2 offspring would have the
following genotypes?

aabbccdd

AaBbCcDd

AABBCCDD

AaBBccDd

AaBBCCdd

10. In 1981, a stray black cat with unusual
rounded curled-back ears was adopted by a family in California. Hundreds of
descendants of the cat have since been born, and cat fanciers hope to develop
the "curl" cat into a show breed. Suppose you owned the first curl cat
and wanted to develop a true breeding variety.

How would you determine whether the curl allele
is dominant or recessive?

How would you select for true-breeding cats?

How would you know they are true-breeding?

11. What is the probability that each of
the following pairs of parents will produce the indicated offspring (assume
independent assortment of all gene pairs?

AABbCc x aabbcc ----> AaBbCc

AABbCc x AaBbCc -----> AAbbCC

AaBbCc x AaBbCc -----> AaBbCc

aaBbCC x AABbcc ----> AaBbCc

12. Karen and Steve each have a sibling
with sickle-cell disease. Neither Karen, Steve, nor any of their parents has the
disease, and none of them has been tested to reveal sickle-cell trait. Based on
this incomplete information, calculate the probability that if this couple
should have another child, the child will have sickle-cell anemia.

13. Imagine that a newly discovered,
recessively inherited disease is expressed only in individuals with type O
blood, although the disease and blood group are independently inherited. A
normal man with type A blood and a normal woman with type B blood have already
had one child with the disease. The woman is now pregnant for a second time.
What is the probability that the second child will also have the disease? Assume
both parents are heterozygous for the "disease" gene.

14. In tigers, a recessive allele causes an
absence of fur pigmentation (a "white tiger") and a cross-eyed
condition. If two phenotypically normal tigers that are heterozygous at this
locus are mated, what percentage of their offspring will be cross-eyed? What
percentage will be white?

15. In corn plants, a dominant allele I
inhibits kernel color, while the recessive allele i permits color when
homozygous. At a different locus, the dominant gene P causes purple kernel
color, while the homozygous recessive genotype pp causes red kernels. If plants
heterozygous at both loci are crossed, what will be the phenotypic ratio of the
F1 generation?

16. The pedigree below traces the
inheritance of alkaptonuria, a biochemical disorder. Affected individuals,
indicated here by the filled-in circles and squares, are unable to break down a
substance called alkapton, which colors the urine and stains body tissues. Does
alkaptonuria appear to be caused by a dominant or recessive allele? Fill in the
genotypes of the individuals whose genotypes you know. What genotypes are
possible for each of the other individuals?

17. A man has six fingers on each hand and
six toes on each foot. His wife and their daughter have the normal number of
digits (5). Extra digits is a dominant trait. What fraction of this couple's
children would be expected to have extra digits?

18. Imagine you are a genetic counselor,
and a couple planning to start a family came to you for information. Charles was
married once before, and he and his first wife had a child who has cystic
fibrosis. The brother of his current wife Elaine died of cystic fibrosis. What
is the probability that Charles and Elaine will have a baby with cystic
fibrosis? (Neither Charles nor Elaine has the disease)

19. In mice, black color (B ) is dominant
to white (b ). At a different locus, a dominant allele (A ) produces a band of
yellow just below the tip of each hair in mice with black fur. This gives a
frosted appearance known as agouti. Expression of the recessive allele (a )
results in a solid coat color. If mice that are heterozygous at both loci are
crossed, what will be the expected phenotypic ratio of their offspring?

20. The pedigree below traces the
inheritance of a vary rare biochemical disorder in humans. Affected individuals
are indicated by filled-in circles and squares. Is the allele for this disorder
dominant or recessive? What genotypes are possible for the individuals marked 1,
2, and 3.